The invention of the present application enables detection of the rate of convective cooling of an electrical sensor. The basic structure of the convective cooling rate sensor of this invention is a silicon bulk resistor which has a positive temperature coefficient of resistance. This silicon bulk resistor is electrically connected to and physically supported by a pair of electrodes. This sensor can be made very small, thereby reducing the thermal mass of the sensor and decreasing the response time. The cross sectional area of the electrodes, or of at least one portion of each of the electrodes, can be selected to provide a desired rate of conductive cooling of the sensor through the electrodes. The convective cooling rate is measured by observing the sensor temperature while adding heat to the sensor. This heating of the sensor is accomplished by ohmic self heating from a predetermined amount of electric power applied to the sensor via the electrodes. The temperature of the sensor is then determined by the relative rates of this electrical self heating and of the thermal cooling of the sensor. That is, the heat retained by the sensor, as indicated by its temperature, is a function of its heat gain and heat loss. The sensor temperature is measured by measuring its electrical resistance, which can be accomplished during ohmic self heating, and referencing the known temperature/resistance characteristic of the sensor. Because the conductive cooling of the sensor through the electrodes is set at a predetermined level by the cross sectional area of the electrodes, the remaining cooling factor, representing the convective cooling due to the fluid surrounding the sensor, can be determined. The convective cooling rate sensor may be employed in thermal equilibrium in which the rate of heating and the rate of cooling are equal and the temperature of the sensor is constant. The convective cooling rate sensor may also be employed in thermal disequilibrium in which the rates of heating and cooling are not equal and therefore the temperature of the sensor is changing. In such a case, the rate of temperature change of the sensor must also be considered in order to determine the convective cooling rate.
The invention of the present application may be employed in a fluid flow rate sensor. It is known that the convective cooling rate of a hot body is dependent on the rate of fluid flow past the hot body. If the temperature of a convective cooling rate sensor disposed in the fluid flow is set at a predetermined level, for example, a predetermined number of degrees hotter than the fluid, then the amount of ohmic self heating necessary to maintain that temperature is dependent on the rate of convective cooling and hence on the fluid flow rate.
Previous hot body fluid flow sensors have employed hot wire sensors. Construction of these hot wire sensors required making an extremely thin wire which was difficult and which yielded a rather delicate structure which could be easily damaged. In addition, the temperature resistance characteristics of such thin wire sensors were unstable requiring frequent recalibration due in part to deposition of contaminants on the sensor. In contrast the structure of the present invention is much more robust, exhibits a greater stability and is more easily mass produced.
The invention of the present application may also be employed as a liquid level sensor. The principle of operation of the present invention when employed as a liquid level sensor is to determine whether a sensor is surrounded by gas or liquid or to determine which of two liquids surrounds the sensor by determining the external thermal load upon the sensor. The present invention employs a temperature sensitive resistor composed of a doped silicon bulk resistor operating in the extrinsic region. A predetermined amount of electrical power is applied to this temperature sensitive resistor. The external thermal load applied to the temperature sensitive resistor is determined by detecting the rise in temperature of this resistor due to the applied electric power. If the temperature sensitive resistor is surrounded by a gas, there is a smaller thermal load imposed upon this resistor than if the same resistor were surrounded by a liquid. That is, a gas would absorb less of the heat energy within the temperature sensitive resistor via convection than would the liquid. As a consequence, for a given amount of electric power applied to the temperature sensitive resistor, the resistor would reach a greater temperature in gas than in a liquid. A similar condition would occur if the resistor could be immersed in one of two immiscible liquids having differing thermal conductivities. The temperature of the temperature sensitive resistor can be determined by measuring its resistance. Thus, the convective cooling rate sensor of the present application enables a completely electrical determination of whether a particular liquid covers the sensor or not.
There have been previously proposed in U.S. Pat. No. 3,412,610, issued to Prussin, Nov. 26, 1968, a silicon temperature sensitive resistor detecting the presence of a liquid by utilizing the extrinsic to intrinsic resistance switching characteristics of semiconductor material. This level sensor had several disadvantages. The material dopant level must be selected for a specific relatively narrow temperature range of application. These sensors were difficult to construct due to the need for high temperature alloys involved in the contact system. This device typically required a minimum of three seconds in order to determine the presence of absence of a liquid surrounding the sensor. Lastly, these devices were unreliable and subject to a shortened device life due to the high temperatures involved in their operation and generally exhibited non-uniform response from device to device. The present invention differs from this prior device in that the extrinsic positive temperature coefficient range is utilized throughout the operation of the device. This type of construction does not require close matching of the dopant level, and therefore the extrinsic to intrinsic switching temperature, to the particular application required. The necessity for high temperature alloys is eliminated. The typical response time of the present device is less than one second. The device life is increased due to reduced operating temperatures. Lastly, greater device uniformity is achieved.